The differentiation of T and B lymphocyte subpopulations, both before and after stimulation with T-dependent or T-independent antigens, is being studied. Particular attention has been paid to the state of reactivity of neonatal mouse lymphocytes. A neonatal T cell subpopulation which suppresses antibody formation has been defined, and two major B cell sublines have been described based upon their reactivity to antigen, surface immunoglobulin expression (surface IgM greater than surface IgD, and vice versa), expression of the Lyb5 differentiation antigen, and presence in the CBA/N mutant subline of mouse. BIBLIOGRAPHIC REFERENCES: Mosier, D.E., Ontogeny of T cell function in the neonatal mouse. In Development of Host Defense, ed. D.H. Dayton. 1977, pp. 115-121. Mosier, D.E., Mathieson, B.J. and Campbell, P.S.: Ly phenotype and mechanism of action of mouse neonatal suppressor T cells. J. Exp. Med. 146: 59, 1977.